Full electroweak supersymmetric quantum effects on Rb in the MSSM

We address the computation of Γz and of the intriguing quantity Rb in the MSSM including full treatment of the Higgs sector. Contrary to previous partial approaches, and due to the possible relevance of the result to the fate of the MSSM, we perform a complete calculation, without approximations. For a pseudoscalar Higgs mass mA0 > 70 GeV and CDF limits on mt, the bounds on Rb at 1σ level leave no room to the MSSM to solve the “Rb crisis” for any combination of the parameters, not even admitting the possibility of a light chargino and a light stop of O (50) GeV; however, for mt not restricted by CDF, a “tangential” solution exists in the window 2 < tanβ < 10 with a light chargino and stop. In contrast, for a pseudoscalar mass 40GeV ≲ mA0 < 60GeV and CDF limits on mt, the “Rb crisis” can be solved in a comfortable way, for any SUSY spectrum above the phenomenological bounds, provided tanβ ≳ mtmb. Our general conclusion is that, if there is a “Rb crisis” at all, its solution within the MSSM has to do more with the peculiar structure of the SUSY Higgs sector rather than with the spectrum of genuine supersymmetric particles. In view of the range predicted for mA0, LEP 200 should be able to definitely settle this question.